The present invention relates to antennas for receiving GPS signals. In particular, the present invention relates to GPS antennas that are optimized for use in proximity to a human body.
Navigation is key to national and international industry, commerce, and safety. Knowledge of position, both relative and absolute has been used throughout history to gain tactical advantage in both peaceful and not so peaceful pursuits. From the rudimentary techniques developed over two millennia ago, people all over the world have made both evolutionary and revolutionary progress in the business of knowing their position. Navigation progressed from simple pilotingxe2x80x94the art of connecting known pointsxe2x80x94to satellite-based navigation systems.
Today the premier worldwide navigation solution is the Global Positioning System (GPS). This satellite-based navigation system was developed by the Department of Defense (DoD) to support a variety of military operations. This system has been used in a variety of civilian systems. As the adoption of satellite-based navigation technology has grown since its introduction in the early 1980""s, so has the number and complexity of devices for personal navigation and location. GPS is broken down into three basic segments, as follows: 1) spacexe2x80x94comprising the satellites; 2) controlxe2x80x94incorporating tracking and command centers; and 3) userxe2x80x94performing navigation functions based on ranging to the satellites.
The space segment contains the GPS Space Vehicles (SVs) placed in circular orbits with 55xc2x0 inclination and a semi-major axis of 26,560 km (20,182 km altitude) corresponding to an orbital period of 12 hours sidereal. There are six orbit planes placed at 60xc2x0 offsets in longitude with nominally four satellites in each plane, giving 24 satellites. Currently there are 28 active satellites in the planes. Spacing within the plane is adjusted to achieve optimal coverage over regions of interest. The satellites themselves are three-axis stabilized and use solar panels to provide power. Each satellite contains a pair of atomic clocks (for redundancy) which have a stability of 1 part in 1013. Each satellite broadcasts on two frequencies, 1575.42 MHz (L1) and 1278.6 MHz (L2). The L1 signal contains two separate pseudo-random noise (PRN) modulations: 1) the Clear Acquisition (C/A) code at bit or xe2x80x98chippingxe2x80x99 rate of 1.023 MHz (i.e., each millisecond there are 1023 modulated bits or xe2x80x98chipsxe2x80x99 transmitted); and 2) the so-called xe2x80x98Pxe2x80x99 code which has a chipping rate of 10.23 MHz or 10 times that of the C/A code. The L2 signal only contains the P code. GPS uses a PRN coding sequence of bits that have a specified length but have the property that different codes do not strongly correlate with one another (i.e., they are orthogonal). The C/A code is 1023 chips long and thus repeats every 1 millisecond. The full P code length is 38 weeks but is truncated to 1 week.
The control segment is responsible for the operation and maintenance of the GPS. There are five monitoring stations worldwide at Kwajalein, Hawaii, Colorado Springs, Diego Garcia and Ascension. These stations measure the discrepancies between the satellite state information (satellite positions and clock) as well as health of the satellites. The Master Control Station (MCS) in Colorado Springs formulates predicted values and uploads them to the satellites. This data is then included in the new message for broadcast to the users.
The user segment comprises GPS receivers that decode the satellite messages and determine the ranges to at least four GPS SVs to determine 3-dimensional position and the receiver clock offset. Users breakdown into two main groups: authorized and unauthorized. Authorized users have full access to both the C/A and P codes. Authorized users are restricted to the military and other special groups or projects with special permission from the DoD. Unauthorized users generally cannot access the P codes as the code itself is encrypted before broadcast by a process known as anti-spoofing (AS). This makes the process of emulating a GPS signal to the authorized user more difficult. The encrypted modulated signal is known as Y code. Additionally the hand-over-word (HOW) between the C/A and Y code is also encrypted. Authorized users are given a xe2x80x98keyxe2x80x99 that allows for the decryption of the HOW as well as the Y code. Authorized user receiver equipment with dual frequency code access uses what is known as the Precise Positioning Service (PPS).
GPS receivers are very sensitive devices capable of measuring the low signal levels available on, or near, the surface of the Earth. A GPS receiver design incorporates radio-frequency (RF) elements, signal downconversion, signal sampling, digital signal processing, as well as computational devices and methods. The first element of the GPS receiver that interacts with the satellite signal is the antenna. The antenna is a RF component that converts the signal present in the air to an electrical signal which is processed by the receiver.
There are many aspects that are important in antenna design that include, but are not limited to, the following: 1) frequency or frequencies of maximum sensitivity; 2) polarization; 3) size; 4) shape; 5) bandwidth; and 6) gain pattern. Depending on the goals of a particular GPS receiver, various antenna design aspects are emphasized or de-emphasized.
Given the above general background of GPS, a variety of GPS receivers have been developed to fill various market niches. One of these markets is personal GPS.
The idea of using a device on or near the human body that is capable of receiving and processing global positioning system (GPS) signals is impractical for the current state of the art. Such a prior art device, if comprised solely of prior-art components, would experience significant difficulty in receiving clear and processable GPS signals. Such difficulty is directly attributable to the fact that the antenna of such a device would be excessively sensitive to gain variations when in the proximity of a human body. In addition, such a prior-art antenna that may incorporate patch elements or micro-strips may be excessively sensitive to the location of a GPS signal source.
The above description relates to problems and disadvantages relating to tracking, logging, and analysis of personal activities, such as position determination of a user of a cellular telephone. These problems can also be seen for blockage conditions inside of cars or trucks as well as other vehicular applications. Other obstructions such as building or trees can have their influence lessened by this novel device as well.
In accordance with the present invention, an antenna arrangement for a GPS signal processing device having a circuit board is disclosed.
In a preferred embodiment of the invention, the arrangement comprises an antenna member mounted to the circuit board. The antenna member includes a first surface, a second surface and a third surface. The third surface adjoins the first and second surfaces. The first, second and third surfaces define a cavity within which is disposed dielectric material. At least one conductive connector comprising first and second ends is in communication with the antenna member first surface. An amplifier is in communication with each conductive connector second end.
The relatively compact size of the cavity antenna design allows for the incorporation of the antenna into a small device that can be worn on or carried in close proximity to the body of a user. This type of antenna is not as sensitive to gain variations when in the proximity of a human body. In fact, the performance of the antenna""s gain pattern can be tuned using the assumption that it is close to the human body. Further, this type of antenna is virtually omni-directional, i.e., it is not problematically sensitive to the location of the GPS signal source. Moreover, the design is such that the antenna arrangement can be oriented within a device in a way that maximizes the number of GPS satellites tracked.